The invention relates to the enhancing of the reliability of a matrix of electronic elements and to a method for locating a fault in the matrix. The invention is especially useful in imaging devices and more particularly in a detector allowing image capture.
This type of device comprises a great number of sensitive dots called pixels generally organized as a matrix or strip. In an image detector, a pixel is the basic sensitive element of the detector. Each pixel converts an incident signal to which it is subjected, such as for example electromagnetic radiation, into an electrical signal. The electrical signals issued by the various pixels of a matrix are collected in succession during a read-out phase so that they can be processed and stored to form an image. The signal from each pixel can be digitized either inside the pixel, in which case the pixel delivers digital information, or digitization can also be performed downstream of the pixel, the pixel then delivering analog information.
The pixels are for example made up of a photosensitive area delivering a current of electric charges as a function of the stream of photons it receives, and an electronic circuit for processing this current. The photosensitive area generally includes a photosensitive element, or photodetector, which can for example be a photodiode, a photoresistor or a phototransistor. The photodetector is connected to an electronic circuit inside the pixel, i.e. arranged upstream of a read bus, the latter being able to collect information from adjacent pixels. Generally, connection between the pixel and the read bus is controlled in such a way that the reading of each pixel is organized. Thus, the pixel comprises a photodetector as well as electronic elements arranged upstream of said connection. There exist photosensitive matrices of large dimensions which can possess several million pixels.
A radiation detector can be used for imaging ionizing radiation, and particularly X- or γ-radiation, in the medical field or in the area of non-destructive testing in the industrial field, for the detection of radiological images. Photosensitive elements make it possible to detect visible or near-visible electromagnetic radiation. These elements are insensitive, or poorly sensitive, to radiation incident on the detector. For this reason, use is often made of a radiation converter known as a scintillator, which converts incident radiation, for example an X-ray, into radiation in a band of wavelengths to which the photosensitive elements in the pixels are sensitive. An alternative consists in producing the photosensitive element from another material performing the direct conversion of the X-ray into electric charges. Such is the case, for example, of matrices in which a first pixellated substrate made of cadmium telluride (CdTe) is connected pixel-by-pixel to a CMOS read-out circuit, which therefore no longer has a detection function.
Each pixel is made up of a block of electronic components which can be relatively complex. This block is linked to arrays of rows and columns of the matrix. These rows and columns guarantee the necessary bias, the control, and the output paths for the information detected by the pixel.
Faults in a matrix may affect either the arrays of rows and columns (short-circuits or open circuits) or the inside of the blocks of electronic components.
The risk of failure of a block increases when so-called critical components are incorporated into the pixel, i.e. components having a higher risk of failure than more standard components. These could be, for example, components making use of certain particular technological processes, or employing particular surfaces: for example, a high-capacitance capacitor can employ thin dielectric films, which may present local defects, or even be of large area, in which case it then runs the risk of being affected by dust during fabrication. Naturally, by increasing the number of pixels, the risk of a pixel failure increases.
To reduce this risk, it is possible to design components or connections inside the blocks with margins of safety over the minimum design rules. However, this limits the number of acceptable components per pixel and therefore the achievable operations.
One can nonetheless tolerate failures in isolated pixels. For example in an image detector, when a pixel is faulty, it is possible to reconstruct the missing information by averaging the items of information issued by neighbouring pixels.
However, failure in an isolated pixel, for example a short circuit, can pollute neighbouring pixels of a row or column, or even prevent the overall operation of the matrix.
To avoid this contagion efforts have been made to locate the defective pixels in order to isolate them by cutting the connections linking the affected blocks to the row or column buses, generally by laser fire. However, this technique has several limitations.                It is an additional technological step, requiring equipment and time, and therefore incurring a cost.        The use of laser fire presupposes knowledge of the position of the faulty pixel. However, in the case where the fault triggers the breakdown of a power supply, the fault before correction can trigger loss of functionality of a large part of the matrix, or even the entire matrix. Location is then difficult or impossible.        The use of laser fire requires the provision of spaces dedicated to laser cutting. This therefore takes up space in the pixels.        Laser fire can be used in the factory, but in practise is not feasible after-sale. It cannot therefore be used to combat faults appearing during the lifetime of the device.        
As an alternative to laser fire, fuses incorporated into each pixel may be used. The limitations are broadly the same as for laser fire.